Its way easier to put graveyard hate in your 75 cards than piles of Doom Blades, and for that hate to be good against Regent it needs to be efficient, hit a bunch of cards, and importantly have fair value
Ideally side board but there are some cards you can run main without it hurting too much like Relic of Progenitus
Finally, Cube is a limited format like Draft except the packs are from a curated collection of cards rather than simply cracking random set packs
The price is enough that you can get decently effective and powerful cards but the cost is also low enough that you can also get a lot of tempo out of using them
Rad counters are a kind of counter that a player may have
Ishgard, the Holy See is always worth including in a white deck, even with only a couple of available targets for the spell, but it gets more desirable the more invested you are in artifacts and/or enchantments